Large-scale pork production is a profitable industry in several regions of the Nation. Unfortunately, this economic success is often at the expense of local ambient air quality. The emission of hazardous and odorous gases from hog manure basins has polarized many rural communities. Public and regulatory pressure applied on the pork industry to control its air emissions already has slowed the expansion of the industry in certain regions. This has potentially severe economic consequences on rural and state economies. The continued economic viability of the regional pork industry and associated rural communities requires that pork production facilities adapt air pollution control technologies that ensure sustainable development. The same need for air pollution control exists in other industries with high-strength organic waste streams, such as the sugar refining, dairy, cannery, and food processing industries.
Modern, large-scale pork production is seldom cited as an example of sustainable development, because ofthe external and often unpaid costs associated with potential adverse effects on local water and air resources. The present trend is that the large-scale pork production facilities provide an increasingly greater fraction of the nation's pork production. The spatial concentration of hogs and hog manure in such facilities results in the release of hazardous and odorous gases that exceed the natural ability of the atmosphere to maintain by dispersion concentrations below acceptable levels. Hydrogen sulfide emitted from outdoor hog manure basins can be detected by substantial portions of the population for considerable distances downwind of the basin. The lack of cost-effective technologies to control the emission of hazardous and odorous gases limits the growth of hog industry at a time when the demand for pork products is increasing. These constraints may result from tougher zoning regulations or fear of litigation. For example, the Sep. 12, 1994 issue of Farmweek magazine published a story describing how neighbors of an 800-hog operation in Iowa were awarded $45,000 in damages because ofnoxious odors, and another story describing how public concerns over hog odors in North Carolina forced county zoning boards to prohibit hog operations in certain locations (e.g. within one mile of towns). Thus, the spatial concentration of hogs and hog manure into larger operations results in a potential reduction in local air quality, which has both public health and public nuisance components.
With the exception of North Carolina, currently most of the pork production in the United States occurs in the corn belt states and parts of their immediate neighbors. Table 1 indicates the monetary value of pork production to the central United States. The crop farmers of this region depend directly and indirectly on the pork and other livestock industries. Midwestern farmers have invested in pork production facilities on their own properties or as part of cooperatives to provide income when crop prices are low. However, the present inability to control the emission of hazardous and odorous gases threatens the livelihood of region's pork industry and the economic viability of rural communities. Such a loss of pork production would represent a significant loss to the Upper Midwest economy in terms of those activities involved directly (Table 1) and indirectly (corn and soybean growers, meat packers, distribution) with pork production.
TABLE 1 ______________________________________ Pork production and direct market value for the 10 major pork produciug states. Head marketed Estimated Value Rank State (In 1000's)a (US $ in millions)b ______________________________________ 1 Iowa 24,160 3,020 2 North Carolina 11,455 1,432 3 Minnesota 9,724 1,216 4 Illinois 9,462 1,183 5 Indiana 7,846 981 6 Nebraska 7,346 918 7 Missouri 6,071 759 8 Ohio 3,623 453 9 South Dakota 3,329 416 10 Kansas 2,404 301 ______________________________________
Aside from aerobic biological treatment, few (if any) technologies have been proven effective in reducing the emission of hazardous air pollutants and odorous compounds from hog manure facilities under diverse environmental and managerial conditions. Part of the problem in developing manure additives for odor control is that the specific compounds responsible for mal-odors have not been satisfactorily identified. Without knowing what the chemical targets are for odor control, the end result is an essentially blind development of odor control products that are only partially effective (at best) in controlling odors.
The anaerobic biodegradation of hog manure generates several general classes of compounds that are considered hazardous, odorous, or both. These classes compounds include ammonia, reduced sulfur compounds (hydrogen sulfide, mercaptans), volatile fatty acids (n-butyric acid, valeric acid), phenolic compounds (phenol, para-cresol), indoles (skatole, indole), and volatile amines (putrescine, cadaverine). In general, the more offensive odorous compounds are associated with the anaerobic biodegradation of proteins. Because of the wide range in chemical properties represented by these classes of hazardous and odorous compounds, no one additive or process can be expected to control all offensive odors--that is expect for aeration. Aeration and the associated aerobic biological treatment can result in the biodegradation of all the above hazardous and odorous gases.
High operating costs are currently associated with aerating large volumes of high strength wastes. It has been suggested that aerobic biological treatment should only be used in situations where odor control is essential, because of the high operating costs. The cost for aerobic biological treatment of the entire manure stream generated by a 150 lb finishing pig is estimated to be about $7.00 per marketed pig. This high operating cost for complete aerobic treatment of hog manure continues to limit its acceptance by the pork production industry.
One means of reducing the costs of aeration is to reduce the volume of water that is aerated by operating outdoor hog manure basins as facultative lagoons. Facultative lagoons have been used in the municipal and industrial waste water treatment industries for decades as a means of meeting treatment objectives with reduced aeration costs. A facultative lagoon is one that has an aerobic (oxygen present) layer above an anaerobic (no oxygen present) layer. The offensive hazardous and odorous gases generated in the anaerobic layer are oxidized by the aerobic (oxygen-requiring) bacteria found in the upper layer of a facultative lagoon and converted into inoffensive products. However, as shown in Table 2, the operating costs associated with using the traditional waste water treatment approach to a facultative lagoon are still too expensive for odor control at most pork production operations.
TABLE 2 ______________________________________ Estimated aeration costs for the aerobic biological treatment of hog manure. Aeration Operation Costs Treatment Option ($/hog marketed) ______________________________________ aerobic treatment of the entire hog manure basin 7.00 facultative lagoon as traditionally operated in 3.70 the wastewater treatment industry ideal facultative lagoon for odor control with 1.50 biological ammonia oxidation in the upper aerobic layer ideal facultative lagoon for odor control without 0.25 biological ammonia oxidation in the upper aerobic layer ______________________________________
Ideally, the operation of the facultative lagoon designed primarily for odor control should supply just enough aeration to biodegrade the hazardous and odorous gases that would otherwise be emitted into the atmosphere. Such an approach is feasible, because many ofthe offensive organic and inorganic gases released during the anaerobic decomposition of hog manure are preferentially biodegraded under aerobic conditions compared to the other components that make up the bulk biological oxygen demand of hog manure. Because the ideal facultative lagoon does not need to maintain sufficient dissolved oxygen concentrations to encourage nitrification, the oxygen demand associated with the biological oxidation of ammonia to nitrate can be removed from the oxygen requirement. Without nitrification, the aeration cost of odor control with the ideal facultative lagoon is $0.25/hog marketed.
As shown in Table 2, a facultative lagoon designed for odor control is significantly more cost effective than the other aerobic biological processes and has a theoretical minimum aeration cost of $0.25/hog marketed. This minimum ideal cost can be approached by aeration equipment that maintains redox potential of at least +10 mV Eh in an aerobic mixing zone that is no greater than one foot thick.